The electrical supply industry uses many transformers to change the voltage of the supply for transmission, improving the efficiency of the transmission network. The transformers most commonly use an insulating oil and cellulose to insulate and separate the windings, the cellulose quickly becoming saturated with the insulating oil shortly after the oil is added. Sometimes the transformers are placed under a partial vacuum prior to the oil addition to speed this process up. The oil is therefore intimately in contact with all of the conductors and any reduction in its insulating or dielectric properties can have detrimental, if not catastrophic, effects. The efficiency may drop or the oil may cease to be an effective insulator resulting in a flashover.
One of the common contaminants that affects the properties of the cellulose/oil and oil properties is water. High water content in the oil or cellulose can:                1. Reduce the dielectric properties of the oil and oil/cellulose.        2. Reduce the insulating properties of the oil and oil/cellulose.        3. Accelerate the breakdown of the cellulose.        4. Exceed saturation point of oil when oil is cooled.        5. Increase corrosion of exposed metal.        
The cellulose starts off at below 1% water content but over time leaks in the cooling system, cellulose breakdown and breather desiccant failure/overrun leads to concentrations above this. At present the industry aims to keep the water content in the cellulose between 1% to 3%, with it generally accepted that over 95% of the water within the transformer is in the cellulose. The water concentration of the circulating oil is in equilibrium with the cellulose water concentration, thus any reduction in the oil's water concentration, over time, reduces the cellulose water concentration.
For this reason the oil circulating through transformers is passed through filtering units, that filter and dry the oil. These filtering units may contain dry cellulose, desiccants such as silica gel or acrylic beads, molecular sieves, activated alumina or other means to remove the dissolved or free water and some form of particulate filter. These filtering units eventually become saturated with water and need replacement, refurbishment, regeneration or drying.
Regeneration of the filtering unit can involve the direct exposure of the filter media to a vacuum under either ambient or elevated temperatures to directly evaporate the water. This can detrimentally affect the pore size and/or surface properties of the media, reducing the refurbished filter's effectiveness or life.
As an alternative the transformer oil may be directly dried by spraying the contaminated oil into a vacuum chamber, this replaces the drying action of the filtering media and can require the vacuum system be inline continuously. This can be an expensive exercise and adds another component that requires maintenance; in addition a particulate filter is still often needed. In addition the oil can be damaged from continuous exposure to high levels of vacuum
For printing the concentration of water in the ink can affect the print quality and longevity of the inks and printing equipment. The high cost of many inks makes controlling this water content important.